a) Field of the Invention
This invention relates to sensors for detecting components of a gas or vapor. More particularly, it relates to detection of hydrocarbon components from a gas chromatographic column. Even more particularly, it relates to an improved thermoelectric device design for detecting an analyte undergoing a catalyzed endothermic reaction.
b) Discussion of the Prior Art
The use of heat sensing devices, such as thermocouples, to detect temperature and temperature changes is well known in the art. Associating a catalyst with the hot junction of a thermocouple to improve its output is also known in the art, for example see Tracht U.S. Pat. No. 3,070,645, in which a catalyst was added to the hot junction of each of the thermocouples in a thermopile in a device to directly convert thermal energy into electrical energy.
Thermocouple systems have also been considered for use as automobile exhaust gas analyzers, operating on the principle that heat generated by completely oxidizing residual combustibles in automobile exhaust gases is proportional, within certain limits, to the concentration of residual combustibles in the automobile exhaust gases. In this regard, and of perhaps greatest interest to the present invention is, U.S. Pat. No. 3,906,721 to Micheli, et al teaches a temperature detector in which the hot and cold junctions of a thermocouple are embedded in a U-shaped ceramic body and mounted in the exhaust system of an automobile. An automobile exhaust system places particular requirements on this type of sensor, because the exhaust gases cool rapidly as they move away from the engine. As a result of this requirement, the Micheli patent describes a device wherein both junctions of the thermocouple are formed in a single U-shaped ceramic body, and this body is placed in the exhaust system perpendicular to the flow of exhaust. Therefore, the exhaust gases which pass both sides of the thermocouple are at the same distance from the engine at the same time, so both junctions of the thermocouple remain at the same temperature, unless one junction reacts with the gases. The difficulty of this system consists of the expense of manufacturing both thermocouple junctions simultaneously, and the requirement that both thermocouple junctions contact the exhaust gases at the exact same point in the exhaust system. The ceramic U-shaped element also makes this detector physically large. Furthermore, since both junctions of the thermocouple are within the stream of the exhaust gas they are both subject to corrosion and abrasion by the gas.
The device of Micheli U.S. Pat. No. 3,906,721 cannot be easily applied as detector for use with a gas chromatograph. In chromatographic detection, analytes are often present at extremely high dilution, and therefore the size of any detection cell used must be carefully minimized, while maintaining good sensitivity. Additionally, the configuration and size of the device of Micheli U.S. Pat. No. 3,906,721 is much too large to be applied to chromatographic detection, and could not be reduced to a useful size without producing a much more complex device, By contrast, and as detailed below, the size of the detection cell used in the device of the present invention is extremely small and inherently simple.
The catalyst used in the device of Micheli U.S. Pat. No. 3,906,721 is completely different from the catalyst used in the device of the present invention. In the device of the Micheli patent, a platinum based catalyst is used since such a catalyst optimizes the oxidation process which the device of the Micheli patent monitors.
Furthermore, the mechanism of operation of the device of Micheli U.S. Pat. No. 3,906,721 is completely different from the mechanism of operation of the device of the present invention. The device of the Micheli patent depends upon catalytic oxidation in which two chemical species, a hydrocarbon molecule and an oxidant molecule, combine in an exothermic reaction to form a new chemical species. In the device of the present invention a single chemical species, a hydrocarbon molecule, by itself, undergoes endothermic catalytic cracking to form two or more hydrocarbon species. The device of the Micheli patent requires that more than one chemical species be present in the process stream, for example a hydrocarbon and an oxidant. Therefore, the device of the Micheli patent would be ineffective in analyzing the gas stream which leaves the typical chromatographic column, since such a gas stream consists of a single, separated gas component eluant in an inert gas carrier. Oxidants are substantially never present in such a chromatographic eluent stream with hydrocarbons, and if they were, the presence of oxidant in a predominantly hydrocarbon stream would at best impair chromatographic operation, and, at worst, could destroy apparatus components and create a potentially explosive hazard. In hind site, in order to render the device of the Micheli patent suitable for use as a detector for chromatography, would require that a controlled concentration of oxidant be added to the process stream, which is neither suggested by Micheli, nor a routine change, in such a system because of the hazards, and which further is not believed to be possible using state-of-the-art instrumentation.
For completeness of this discussion, it should be noted that, while there are some chromatographic detectors which require the application of an oxidant for operation, the oxidant is applied outside of the eluent or process stream, and therefore does not dilute the eluent or process stream.
It is thus seen that it would be desirable to have an improved thermoelectric detection system which resolves these problems.